The present invention relates to a heat-resistant porous article having fine cells and a low dielectric constant and to a process for producing the same. This porous article is highly useful as, for example, a circuit substrate for electronic appliances, etc.
Because of their high insulating properties, plastic films have conventionally been utilized as parts or members required to have reliability, such as circuit substrates and substrates for printed wiring boards, in electronic/electrical appliances, electronic parts, etc. In the field of electronic appliances where a large quantity of information is stored and is processed and transmitted at a high speed so as to cope with the recent highly information-oriented society, plastic materials for use therein are also required to have higher performances. In particular, a lower dielectric constant and a smaller dielectric loss tangent are desired as electrical properties necessary for the use of higher frequencies.
The dielectric constant of a plastic material is generally determined by the molecular structure thereof. This means that a technique which may be effective in reducing dielectric constant is to modify a molecular structure. However, in view of the fact that polyethylene and polytetrafluoroethylene, which are regarded as low dielectric constant-polymers, have dielectric constants of about 2.3 and about 2.1, respectively, there are limitations in the technique of controlling dielectric constant based on structure modifications.
There is another attempt to reduce dielectric constant by making a plastic material porous to thereby control the dielectric constant of the material based on the porosity thereof so as to take advantage of the dielectric constant of air, which is 1. Various proposals have been made on this technique.
Known common processes conventionally used for producing a porous article include dry processes and wet processes, and the dry processes include a physical process and a chemical process. The general physical process comprises dispersing a low-boiling liquid (foaming agent) such as a chlorofluorocarbon or hydrocarbon into a polymer and then heating the polymer to volatilize the foaming agent and thereby form cells. The chemical process for obtaining a foam comprises adding a compound (foaming agent) to a polymer base and pyrolyzing the compound to generate a gas and thereby form cells.
For example, U.S. Pat. No. 4,532,263 discloses a method for obtaining a foamed polyetherimide or the like using methylene chloride, chloroform, trichloroethane or the like as a foaming agent. However, this foaming technique has various environmental problems such as the harmfulness of the substances used as foaming agents and ozonosphere depletion. In addition, it is difficult to obtain with this technique a foam having fine cells uniform in diameter, although the technique is generally suitable for obtaining a foam having a cell diameter of tens of micrometers or larger. On the other hand, the latter foaming technique, which is a chemical process, has a drawback that a residue of the foaming agent which has generated a gas remains in the resulting foam. Such corrosive gases and impurities pose a problem concerning pollution especially in applications such as electronic parts, where pollution reduction is highly required.
Recently, a technique for obtaining a foam having a small cell diameter and a high cell density has been proposed. This technique comprises dissolving a gas such as nitrogen or carbon dioxide in a polymer at high pressure, subsequently releasing the polymer from the pressure, and heating the polymer to around the glass transition temperature or softening point thereof to thereby form cells. This foaming technique, in which cells are formed by forming nuclei from the gas in a thermodynamically unstable state and then expanding and growing the nuclei, has an advantage that a foam having microporosity which has been unobtainable so far can be obtained.
JP-A-6-322168 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) proposes a process for producing a heat-resistant foam by applying any of those techniques to a polyetherimide as a thermoplastic polymer. However, this process has the following drawback. When a polymer is impregnated with a high-pressure gas in a pressure vessel, the pressure vessel is heated to or around the Vicat softening point of the polymer. Because of this heating, the polymer is in a molten state during pressure reduction and, hence, the high-pressure gas readily expands. As a result, the foam obtained has a cell size as large as from 10 to 300 xcexcm. Consequently, this foam, when used as a circuit substrate, needs to have a large thickness and imposes limits on the formation of finer patterns.
On the other hand, JP-A-10-45936 proposes a technique of forming a foamed molding having closed cells with a cell size of from 0.1 to 20 xcexcm by likewise applying any of those techniques to a styrene resin having a syndiotactic structure, and further proposes use of the foamed molding as an electric circuit member. However, since styrene resins having a syndiotactic structure generally have a glass transition point around 100xc2x0 C., this foamed molding deforms or bends when used at temperatures of 100xc2x0 C. or higher. Consequently, this foamed molding is usable only in a limited range of applications.
Furthermore, JP-A-9-100363 proposes a low dielectric constant insulating plastic film characterized by comprising a porous plastic having a porosity of 10 vol % or higher likewise obtained using carbon dioxide or another substance as a foaming agent and by having a heat resistance temperature of 100xc2x0 C. or higher and a dielectric constant of 2.5 or lower. Although there is a description therein to the effect that an average pore size of 10 xcexcm or smaller is obtainable, the actually attainable minimum cell size is about 5 xcexcm at the most as far as the disclosure therein is viewed. It is therefore expected that there are limits on the formation of finer patterns.
Accordingly, one object of the present invention is to provide a porous article which not only has excellent heat resistance and a finely cellular structure but has a low dielectric constant.
Another object of the present invention is to provide a process for producing the porous article.
As a result of extensive investigations to overcome the conventional problems described above, it has been found that when an additive is added to a heat-resistant polymer such as a polyimide to form a specific micro-domain structure and is then removed therefrom by heating and solvent extraction while utilizing differences between the two ingredients in volatility (boiling point) or thermal decomposability and in solubility in a solvent, a porous article having extremely fine cells and a low dielectric constant is obtained. The present invention has been completed based on this finding.
The invention provides a process for producing a porous article which comprises subjecting a polymer composition having a micro-domain structure comprising a continuous polymer phase and dispersed therein a discontinuous phase having an average diameter smaller than 10 xcexcm to a treatment for removing an ingredient constituting the discontinuous phase by at least one operation selected from vaporization and decomposition and by an extraction operation to thereby make the polymer porous.
The ingredient constituting the discontinuous phase has a weight average molecular weight of, for example, 10,000 or lower. Liquefied carbon dioxide or supercritical carbon dioxide can be used as an extraction solvent for the ingredient constituting the discontinuous phase.
The invention further provides a porous article which comprises a heat-resistant polymer and has an average cell diameter smaller than 5 xcexcm and a dielectric constant of 3 or lower.